Best and worst – mpg

How much could you save on fuel bills by downsizing from a Bugatti Veyron to a Smart 0.8 CDI?

It's unlikely that you'll be making such an extreme switch, but your choice of car can have a dramatic impact on your fuel bills, even if you stick within the same class.

We're going to illustrate the point here in every sector, showing the difference in fuel bills over a typical three-year ownership period with 36,000 miles covered (a distance that a Veyron is highly unlikely to cover in its lifetime).

We'll explore the difference between the best and worst diesel version in each class as well as the best and worst petrol. We'll also show how big the difference in fuel bills are between the best diesel and the worst petrol.

Our comparisons are based on average fuel prices of £1.27 a litre for petrol and £1.31 a litre for diesel.

All of the comparisons are also based on the official average mpg. It's not a perfect system - you're unlikely to get within 10% of the figure in the real world – but at least it's a level playing field.

There's more to life than fuel costsDon't forget, though, there's more to the running costs of your car than fuel. Depreciation plays a massive role in how cost-effective your car will prove to be over three years, so always remember to factor that in, along with insurance, servicing and tax costs.

To help you with your sums we've produced a petrol versus diesel calculator into which you can punch the numbers to find out which car really will cost you less over three years and 36,000 miles.